1. Field of the Invention
The invention relates to an implantable prosthesis. In particular, the invention relates to a self-expanding endoluminal graft material for use with a stent as a stent-graft. The invention is particularly suited for repairing the aortic artery and daughter arteries, although it is not limited thereto.
2. State of the Art
Two types of implantable prostheses utilize tubular graft materials. These prostheses are known as vascular grafts and endoluminal stent-grafts. The endoluminal stent-graft typically includes tubular graft material which is affixed (usually with sutures) to the inside or outside of a woven metallic stent and is delivered to the damaged site via a catheter, whereas the vascular graft does not utilize a stent and is sutured in place using traditional open surgical techniques.
Vascular grafts are most often used to treat aneurysms and are typically made of tightly woven polyester (polyethylene terephthalate--"PET") fibers. PET fibers are chosen because they have a history of satisfactory long term use in the human body and because they can withstand relatively high hoop stress which is imparted by blood pressure in large diameter vessels. The fibers are tightly woven to limit the porosity of the graft in order to prevent blood loss during the initial stages of implantation and to facilitate preclotting with blood. As a result, these grafts are relatively non-compliant tubes which exhibit very little change in dimension when stressed in either the axial or radial directions. To further appreciate the lack of distensibility of these grafts, when using these types of grafts in a joint area, such as across a knee joint, these grafts must be mechanically and thermally crimped or corrugated in order that it be able to flex and change length when the recipient bends the knee joint. Such corrugations or crimps are also helpful during implantation of the graft so that the graft, if inadvertently cut too short, may be axially elongated during implantation. Although well-suited for open surgical procedures these bulky vascular graft constructions are difficult to use in an endoluminal application where the graft must be folded down within a deployment catheter.
An elastomeric vascular graft is disclosed in U.S. Pat. No. 4,475,972 to Wong, the complete disclosure of which is hereby incorporated by reference herein. As disclosed in Wong, polyurethane fibers are drawn from a viscous solution and extruded from a transversing spinnerette onto a rotating mandrel. As the filaments or fibers are wet as they are wound, they bond to each as they dry. The resulting graft is a porous tube having elastomeric properties. While the graft disclosed by Wong would appear to have many advantages, polyurethane grafts are not noted for their strength and it is believed that polyurethane may degrade over time. Therefore, it is believed that the prosthesis disclosed by Wong may be unsuitable for long term use in the human body and is not well suited for treating aneurysms. The graft taught by Wong may have greater elastomeric compliance than the vessel to which the graft is attached. This can result in an aneurism if a portion of the graft balloons after implantation. Thus, when treating aneurysms, the relatively non-compliant PET grafts are generally preferred.
My prior U.S. Pat. No. 5,163,951 discloses an improvement to the Wong graft wherein a PET mesh is adhered to the outside of the elastomeric Wong graft. The PET mesh is woven in a loose knit pattern (e.g. tricot or double tricot warp knit, atlas or modified atlas warp knit, jersey or double jersey patterns, etc.) to provide it with compliance. The PET mesh is adhered to the Wong graft material using an intermediate material (e.g. an aliphatic polycarbonate urethane) which has a melting point substantially lower than both the PET mesh and the Wong graft material. The intermediate material is placed between the PET mesh and the Wong graft material and the three component tubular structure is heated so that the intermediate material melts and mechanically bonds the PET mesh and Wong graft material during cooling. The compliance and porosity of the three component graft may be adjusted by adjusting the knitting parameters, the size and number of strands, and the angle at which the strands are drawn. The PET mesh gives the graft greater load bearing ability and also facilitates retention of sutures within the graft while maintaining some of the compliance of the Wong graft material.
Endoluminal stents are most often used to repair blood vessels affected by a variety of lesions which can compromise circulation of blood through the vessel, i.e. stenoses. A typical prior art stent, shown in FIGS. 1 and 2, is a metallic structure 10 made of braided wire 12 such as stainless steel, cobalt-chromium-nickel super alloys and combinations, co-extrusions or braised combinations of the above with tantalum, gold, platinum and the like. Stents are also made from memory alloys such as nitinol and the like. Typical stents are disclosed in U.S. Pat. Nos. 4,655,771 and 4,954,126 to Wallsten, the complete disclosures of which are hereby incorporated by reference, and in U.K. Patent Number 1,205,743 to Didcott, the complete disclosure of which is also hereby incorporated by reference. Generally, the wires 12 are braided with a large pick size, i.e. with relatively large interstices 14 between the wires, so that axial expansion of the stent causes a diametrical compression of the stent as shown in prior art FIG. 2. Most often the braiding and/or the metal chosen for the wires yields a resilient stent which is self-expanding. The ends of the stent are axially displaced during delivery so that the stent has a reduced diameter and can be easily located in the vasculature via a relatively small catheter. This is important since the location of the stent is typically severely narrowed by the stenosis. Upon locating the stent in the vessel, the stent is released so that the stent self-expands, as shown in prior art FIG. 1, and fixes itself to the interior of the vasculature thereby opening a passageway for blood circulation. While endoluminal stents have been used without any graft material, it is now preferred to use a graft material with the stent in order to prevent the growth of lesions through the picks (voids) in the stent and thus re-stenosis of the vessel.
The graft material most often used in endoluminal grafts is a PET or polytetrafluroethylene (PTFE) material which is folded to reduce its size and which is attached to one or both ends of a radially expandable stent by means of sutures. When the stent self-expands or is balloon expanded, the graft unfolds around the stent. A disadvantage of the non-elastic folded graft material is that it cannot be delivered through a small catheter. It is known to provide a porous endoluminal graft which is made of a spun matrix of polyurethane combined with a self-expanding stent. (See, Karo and Dereume et al., Transactions of the 21st Meeting of the Society of Biomaterials, Mar. 18-22, 1995, San Francisco, Calif., page 81.) The elastomeric polyurethane fibers allow the graft to compress with the stent and thereby permit delivery of the stent-graft through a relatively small catheter. However, as mentioned above polyurethane fibers may degrade over long term use in the human body.
While the primary use of endoluminal stents is to treat stenoses, stents are also sometimes used in conjunction with graft material to bridge aneurysms. The advantage of using a stent in bridging aneurysms is that the expanded stent helps to fix the graft in place, can eliminate the need for sutures, and may provide some additional resistance to hoop stress. As mentioned above, however, the preferred graft material for the treatment of aneurysms is relatively non-compliant tightly woven PET. In order to use this graft material, it must be folded, attached to the stent with sutures, and delivered through a relatively large catheter.